1. Field of the Invention
This invention relates to a scanning device, and, more particularly, it is concerned with a scanning device which scans with light beam an object having a flat reflection surface and an inclined reflection surface with an inclination relative to the flat reflection surface, and detects with a light detector only the reflected light from the inclined reflection surface.
2. Description of the Prior Art
There are many kinds of objects having a flat reflection surface and an inclined reflection surface. In the present specification, explanation will be made of an alignment mark on a mask and a wafer for use in fabrication of IC and LSI, as a sample.
In general, alignment of the mask and wafer has so far been done by moving in parallel any one of the mask and the wafer relative to the other until the alignment marks on both of them come to a predetermined positional relationship. In order to carry out the appropriate alignment, the alignment marks provided on both mask and wafer must be observed.
For the method of observing this alignment mark, there have been known the so-called "bright field observation", in which reflected light from the flat reflection surface is observed, and the so-called "dark field observation", in which the reflected light from the inclined reflection surface is observed. The scanning device of the present invention is concerned with the latter method, i.e., dark field observation, about which more detailed explanations will be made hereinbelow.
A device for observing an object having both a flat surface and an inclined surface, such as the alignment mark of a wafer for IC pattern printing in a dark field, has already been proposed in U.S. application Ser. No. 672,022, filed Mar. 30, 1976, now U.S. Pat. No. 4,062,623 and having the same assignee-to-be as that of the present application. While the device in this earlier application will be explained in detail in reference to a drawing illustration at a later paragraph, it can be outlined as follows. An image of a light source for illuminating the surface of an object and which is smaller in size than the pupil of a telecentric objective lens is formed on the pupil plane of the lens, then the entire region of the object to be observed is illuminated, and the regular reflected light from the flat surface within the region to be observed is intercepted by a light intercepting plate of a size substantially corresponding to the image of the abovementioned light source disposed on the pupil surface of the telecentric objective lens or the image surface of the pupil, whereby an object image to be formed by a non-regular reflected light from the inclined surface within the abovementioned region to be observed and passing through the light intercepting plate (i.e. dark field image) is scanned.
In this specification, the term "telecentric objective lens" refers to a lens having such a property that the principal light ray becomes parallel with the optical axis, when the light source is disposed at an intersection of the optical axis of the lens and its pupil surface. By the term "flat surface", it is meant such surface that intersects orthogonally with the optical axis of the telecentric lens. Also, the term "inclined surface" means such surface that does not intersect orthogonally with the optical axis of the telecentric lens.
As is understandable from the above explanations, the invention as disclosed in the earlier application illuminates the entire region of the object to be observed, and sequentially detects lights from a part of the region, on account of which only a very small portion of the light amount for illuminating the object is used at the time of the light detection. Thus, it cannot be said that the illuminating light is utilized in an effective manner.
The copending U.S. application Ser. No. 790,072 filed Apr. 22, 1977, relates to improvement in the prior invention in the abovementioned earlier application, now U.S. Pat. No. 4,062,623 the point of improvement of which resides in utilizing this illuminating light to the object as effectively as possible. For this purpose, the device concerned adopts the so-called "flying spot light scanning system", in which the object surface is sequentially scanned with a relatively narrow scanning light beam, and the light reflected from this object surface is detected.
It should, however, be noted that the invention disclosed in U.S. Ser. No. 790,072 now U.S. Pat. No. 4,062,623 is not a simple application of the flying spot light scanning system to the device of the abovementioned earlier U.S. application Ser. No. 672,022, but intends to solve a problem which is liable to occur when this flying spot light scanning system is adapted to the device of the prior invention. This problem is derived from the fact that, when the scanning beam is projected into the telecentric objective lens, the position of the scanning beam crossing the pupil plane of this telecentric lens varies due to deflection or swerving of the scanning beam, on account of which the position of the scanning beam crossing the abovementioned pupil surface is varied, when the reflected light from the flat surface travels backward. As the result of this, there takes place such possibility that the reflected light from the flat surface (regular reflected light) becomes unable to be removed due to presence of the light intercepting member disposed on the pupil surface of the telecentric objective lens of the image surface of the pupil.
This problem is solved, according to the abovementioned invention, by making constant the position where the scanning beam crosses the pupil surface of the telecentric lens, i.e. by causing the original point of deflection of the scanning beam to coincide with the pupil plane of the telecentric lens.
The present invention further improves the improved device as mentioned above, in that the scanning beam is moved at a constant speed.